Unlike the case of color television broadcasting, where a single system (the NTSC system) was approved by the Federal Communications Commission (FCC) for use in the United States, the FCC did not establish a single-system standard when it authorized stereo AM broadcasting to begin in 1982. As a result, any system may be used by AM broadcasters so long as it meets certain minimum technical specifications which the FCC imposed. In fact initially four different systems were used by AM broadcasters. These were known as the Harris, Kahn/Hazeltine, Magnavox and Motorola systems. Since then, use of the Harris and Magnavox systems has been effectively discontinued, so that at the time of this writing only the Kahn/Hazeltine and Motorola systems remain in use in the United States. Hence, the need arose for a multi-system AM stereo receiver, which automatically recognizes and selects appropriate stereo signal decoding circuitry for the particular AM stereo system signal being received at the time. Such a receiver must also be compatible with conventional monophonic AM broadcasts. Furthermore, such receivers should preferably be inexpensive, and for this purpose preferably make use of existing integrated circuits (IC's), instead of requiring the development of new custom IC's in order to achieve this purpose.
It is, therefore, an object of the present invention to provide a multi-system AM stereo receiver which is compatible with monophonic AM broadcasts, and which is capable of receiving and properly decoding AM stereo broadcasts using the Kahn/Hazeltine and Motorola systems, but which does so automatically and using existing IC's.